Different types of thermoplastic materials soften at different temperatures or within different temperature ranges. The changes in the physical properties of the thermoplastic materials which take place during softening can be used for the sorting of plastic materials or for the separation of plastic materials from materials such as glass or metal. This problem arises particularly during recycling. For this purpose plastic materials have to be selectively softened corresponding to their type and removed from the mixed stream by appropriate devices.
According to the state of the art thermoplastic components are sorted out from a mixed stream by bringing the mixed stream into contact with a sufficiently heated separation surface, whereby the plastic components adhere to the separation surface due to the melting of their contact zones onto this surface. This removes the thermoplastic articles from the mixed stream and enables them to be subsequently scraped off (DE-37 27 777 C2).
Since this separation principle relies on the adherent behavior of plastic materials to be sorted which are softened to the point where they are almost melted and fluid, it is sensitive to contamination in practice, such as during the recycling of plastic materials. Inherent wetness, residuals of the formerly contained goods or labels can hinder the adhesion. The quantitatively interesting polyolefin plastics are by nature bad adhesives and therefore not very readily separated by this sorting method.
Furthermore in this process during operation the sorting surface is increasingly covered by a layer of softened thermoplastics. This layer renders the process less effective, on the one hand due to reduced heat transmission, and on the other hand due to the fact that as a result of the adhesive action of softened thermoplastic material, plastic components which in themselves are not softened at the respective temperature and therefore are not among the target fraction, can stick to this layer. As a result the purity of the target fraction is questionable.
In the separation process according to WO-A-88/000 98 the thermoplastic components are not softened through selective heating and especially not in temperature zones which are specific to type. The components become adhesive due to the temperature of the metal surfaces they come in contact with and melt on the latter. The device required for carrying out the process is extremely complicated and expensive. It is particularly hard to conceive how the pins or lances rotating together with an eccentrically supported pin roller can be heated to be brought precisely to a certain temperature by the heating device inside a drum, so that previously unheated plastic components can melt on it when passing by.
A further drawback of this device working according to the unsatisfactory principle of separation based on adhesion is that it is necessary to provide here special cleaning devices for the removal of the molten residuals of the plastic components from the heated surfaces. For this purpose a separate burner device for singeing the outer roller surface is used, with subsequent brushing or another kind of removal of the remaining residuals.
U.S. Pat. No. 3,793,424 is based on the principle of separation through adhesion or adhesive behavior of the plastic components. There it is indicated that the dry particles are fed to a heated, moving substrate, to which the plastic materials with a lower softening point adhere and the particles with a higher softening point flow freely over it.
JP-A-54 022 477 can also classified as based on the unsatisfactory principle of separation through adhesion. There, in the center of a collection chamber, is an electrically heated cylinder with a conical end. Through that a plastic component is melted in a punctiform manner and penetrates with the melt into the area of the collection chamber. The process comprises the following steps:
(a) local heating of a limited portion in the thermoplastic material with the conical end of a heating element with high thermal conductivity, PA0 (b) this material is brought into contact with a second element of lesser thermal conductivity with a collection chamber, PA0 (c) continuous heating of the material while a molten portion is introduced into the enlarged space of the collection chamber, and PA0 (d) cooling of the molten plastic material while it is introduced in the collection chamber.
The corresponding device with individual, electrically heated melting bodies and the respective collection bodies, partially traversed by cooling channels, assigned thereto is extremely complicated and prone to failure, very difficult to control from the point of view of temperature setting and not suitable for an economical operation in view of the time-consuming process steps (a) to (d). Consequently the plastic components are not softened in defined temperature zone, but melt on contact with heated bodies.